A general solid polymer fuel cell is comprised of a basic structure of a proton conductive electrolyte membrane, catalyst layers forming the anode and cathode arranged at the two surfaces of the same, further, gas diffusion layers arranged straddling these at the outsides, and, furthermore, separators arranged at the outsides of these. This basic structure is used as a unit cell. Usually, a plurality of unit cells are stacked to obtain the required output.
Further, to take out current from a fuel cell of the above basic structure, oxygen or air or other oxidizing gas is supplied from gas channels of the separators arranged at both the anode and cathode to the cathode side and, further, hydrogen or other reducing gas is supplied to the anode side through the respective gas diffusion layers to the catalyst layers. For example, if utilizing hydrogen gas and oxygen gas, the energy difference (potential difference) between the following chemical reaction occurring on the anode catalyst metal (oxidation reaction):H2→2H++2e− (E0=0V)and the following chemical reaction occurring on the cathode catalyst metal (reduction reaction)O2+4H++4e−→2H2O (E0=1.23V)is utilized to generate power.
Here, as the catalyst of the oxidation reaction and reduction reaction, a catalyst metal particle-supporting carbon material comprised of a supporting carbon material made to physically support catalyst metal particles having a catalytic action has been used. Various studies have been conducted up to now on metals having this catalytic action, but a solid polymer fuel cell operates in a strongly acidic environment, so platinum exhibits the highest reaction activity as pure metal in both an oxidation reaction and reduction reaction. For this reason, platinum alone or an alloy mainly comprised of platinum has solely been used for the catalysts in the energy farms currently engaged in commercial production or in the fuel cell vehicles considered close to able to be practically used. In general, the above oxidation reaction and reduction reaction occur on catalyst metal particles, so to raise the rate of utilization of the catalyst, making the specific surface area per mass of the catalyst metal particles larger is essential. For this reason, catalyst metal particles are generally several nm or more in size.
Further, as the support for supporting the catalyst metal particles, from the viewpoints of the electron conductivity, chemical stability, and electrochemical stability, a carbon material has been used. Further, to raise the supporting ability of the support, that is, to increase the sites for adsorbing and supporting the catalyst metal particles, a carbon material with a large specific surface area has been used. Specifically, a carbon material comprised of particles of a size of several 10s to several 100s of nm or so which are made porous at their surfaces to facilitate adsorption of catalyst metal particles (porous carbon materials) has generally been used.
Further, when forming a catalyst layer, to enable diffusion of the reaction gas through this catalyst layer without resistance, the support must be formed with pores for flow of the gas. As the form of a carbon material realizing this object, a carbon material having “structures branched three-dimensionally” called “tree-branch structures” has generally been used. The carbon material spreading the most at the present point of time is the carbon material called “carbon black”. This carbon black has structures of particles of 10s of nm size connected in tree-branch shapes (aggregate). When forming the catalyst layer, the spaces between the branches become pores for gas diffusion. A large current characteristic requiring a large amount of gas and other superior characteristics are exhibited relating to gas diffusion.
As the carbon material having “structures branched three-dimensionally” referred to here, specifically, Vulcan XC-72 made by Cabot, EC600JD made by Lion, and EC300 made by Lion may be mentioned. When using these to form a fuel cell catalyst layer, a carbon material supporting catalyst metal particles and an electrolyte resin are made to disperse in ethanol or water or another suitable solvent, then the obtained dispersion is coated on a Teflon® sheet or other base material and dried to prepare a catalyst layer. In this catalyst layer, spaces derived from the tree-branch shaped structures of the supporting carbon material and other 3D structures are formed. Furthermore, parts of the carbon material having a 3D structure entangle to form a tree-branch shaped network.
As explained by the above-mentioned principle of power generation, in a solid polymer fuel cell, proton conduction, electron conduction, and flow and diffusion of reaction gas (anode side: hydrogen, cathode side: oxygen) are essential for smooth progression of the oxidation reaction and reduction reaction to generate power. Specifically, unless the gas diffusion paths enabling oxygen gas or hydrogen gas to move from the gas channels of the separators to the catalyst metal particles at the insides of the catalyst layers of the cathode or anode, proton conduction paths enabling conduction of protons (H+) generated on the catalyst metal particles of the anode side to be conducted through a proton conductive electrolyte membrane to the catalyst metal particles at the cathode side, and electron conduction paths enabling conduction of electrons (e−) generated on the catalyst metal particles of the anode side through the gas diffusion layer, separator, and external circuit to the catalyst metal particles of the cathode side are respectively connected in series without break, it is not possible to efficiently take out current.
Therefore, inside the catalyst layer, in general, it is important that the pores forming the diffusion paths of the oxygen gas or hydrogen gas formed at the spaces of the composite material, the electrolyte material forming the proton conduction paths, and the carbon materials or metal materials for separator use and other conductive materials forming the electron conduction paths form respective connected networks.
Further, as the polymer electrolyte material for the proton conduction path in the proton conductive electrolyte membrane or catalyst layer, an ion exchanged resin such as a perfluorosulfonic acid polymer is used. However, in these generally used polymer electrolyte materials, proton hopping through water molecules is the mode of conduction, so high proton conductivity is first expressed in a moist environment, while in a dry environment, the proton conductivity ends up falling. Therefore, to operate a fuel cell so that no loss of output voltage occurs, it is essential that the polymer electrolyte material be in a sufficiently moistened state. Therefore, it is necessary to constantly supply water vapor together with the gas supplied to the cathode and anode electrodes so as to moisten the polymer electrolyte material.
In this regard, the output voltage of the solid polymer fuel cell is at the highest 1V or so in open circuit voltage due to the various overvoltages. If the cathode is this potential or less, it is possible to substantially ignore the oxidation of the carbon material used as the catalyst support and consumption as CO2 gas (deterioration of supporting carbon material). However, under actual operating conditions, for example, at the time of starting up or stopping the fuel cell etc., the voltage sometimes rises to 1.3V or more. That is, if mixing hydrogen and oxygen in the anode electrode, a hydrogen oxidation reaction in which the hydrogen is oxidized and an oxygen reduction reduction in which the oxygen is reduced occur inside this anode electrode and a local battery is formed. The potential of the part inside the anode electrode at which the oxygen reduction reaction occurs becomes an oxygen reduction potential (about 1V), so the potential of the counter electrode facing it across the electrolyte rises to the potential of the oxygen reduction potential in the node plus the the battery voltage and the potential of the cathode is observed to rise to 1.3V or more, in some cases, 1.5V or more. Such mixing of the hydrogen and oxygen at the anode electrode occurs due to the oxygen of the cathode electrode passing through the solid polymer electrolyte membrane and reaching the anode electrode. This phenomenon is in principle unavoidable so long as using a fluorine-based membrane with an oxygen permeability—both for energy farms and other stationary applications and further for applications for fuel cell vehicles (NPLT 1).
If the high potential of the cathode causes oxidation of the supporting carbon material expressed by the reaction of C+O2→CO2 and the supporting carbon material deteriorates and is consumed, the catalyst metal particles detach from the support and the amount of catalyst effectively functioning decreases so the power generation performance falls. Alternatively, consumption of the support causes the catalyst layer to become thinner and collapse of the pores causes feed of gas to be obstructed. Due to this, again, the power generation performance falls. Further, the high potential oxidation of the catalyst metal comprised of the platinum also causes deactivation of the catalyst metal etc., so again the power generation performance falls. Therefore, in an actual usage environment, the cell is repeatedly started and stopped. Each time, the cathode is exposed to a 1.3V or more voltage. If this is repeated over a long period of time, the consumption of the supporting carbon material causes an increased drop in the power generation performance corresponding to the number of times of such potential fluctuation.
Therefore, in the past as well, to eliminate the problems due to such deterioration of the supporting carbon material, use of a support comprised of a metal oxide or metal nitride with a high oxidative consumption resistance has been studied (NPLT 2). However, a metal oxide and metal nitride are generally high in water affinity. If using one as a support to form a catalyst layer, since this support and both the catalyst metal and proton conducting resin forming the catalyst layer have affinity with water, the catalyst layer becomes high in water affinity. As a result, the so-called “flooding” phenomenon where the water vapor generated in the cathode side reduction reaction condenses in the electrodes occurs and a large current can no longer be taken out, so this is not suitable for use as a power source. Further, a metal oxide and metal nitride are poor in electron conductivity inside substances. Further, the contact resistance is also high, so a catalyst layer comprised of a catalyst using these as supports becomes higher in electrical resistance, the voltage loss becomes greater, and it becomes difficult to take out a large current. From this viewpoint as well, this is not suited for practical use.
Further, for the purpose of preventing the contamination of the anode electrode by oxygen, suppression of the oxygen permeability of the solid electrolyte membrane is also being studied (PLT 1). A polyimide other than a fluorine-based membrane having a high chemical stability and having a high mechanical strength even if made thinner has been improved in performance until exhibiting a performance in proton conductivity equal to that of a fluorine-based membrane. However, no solution to the basic issue of the problem of the chemical stability is yet in sight at the present time. The long term operating durability has not yet reached the practical level. In a fluorine-based membrane as well, a composite membrane using a nonproton conductive porous membrane having a high mechanical strength as a matrix and impregnating the pores with a proton conducting resin has been studied. However, there is a tradeoff between the mechanical strength and proton conductivity. No practical solution has yet been obtained at the present time.
At the present point of time, the most effective approach has been study to raise the carbon material in oxidative consumption resistance. The oxidative consumption of the carbon material is a reaction in which the carbon at the end parts of the condensed polycyclic aromatic skeleton (below, called “graphene”) (below, called “edge carbon”) is oxidized and consumed as CO2. Therefore, decreasing the edge carbon leads directly to suppression of oxidative consumption at the carbon material. Specifically, if using a highly crystallized carbon material, that is, a carbon material with large graphene growth, it is possible to suppress the oxidative consumption. In general, it is known that graphene grows and develops at 2000° C. or more and, further, graphene becomes stacked highly orderly at 2400° C. or more. Therefore, up to now, application of a carbon material heat treated at 2000° C. or more in temperature to the catalyst supporting carbon material so as to obtain a catalyst improved in oxidative consumption resistance has been studied (NPLT 3). Further, use of the easily graphitizable carbon material of meso synthetic graphite for a catalyst support has been studied. Specifically, the carbon starting material is heat treated in an inert atmosphere at a 2000° C. or more high temperature (PLT 1).
On the other hand, PLT 2 proposes a carbon material having a basic skeleton of graphene formed and grown in a high temperature environment at the time of an explosive reaction caused by heat treating silver acetylide used as a starting material. In the process of the explosive reaction of this silver acetylide, graphene is produced at a high temperature in a short time, so a carbon material comprised of graphene with even sizes of several nm or so and with few defects can be obtained. This carbon material is provided with the oxidative consumption resistance sought from the supporting carbon material of the fuel cell from the viewpoint that the graphene size is an even nm size and, since acetylide is the starting material, the rate of formation of graphene is high, the amount of amorphous carbon is small, and the amount of edge carbon is small. Further, this carbon material is comprised of graphene, as a source of carbon, produced and grown by an explosive reaction using silver acetylide as a starting material, so basically the formed carbon material does not contain oxygen or hydrogen. For this reason, this carbon material differs from usual carbon material in that there is no edge carbon at the ends of the hydrogen forming the starting points of oxidative consumption or edge carbon to which oxygen-containing functional groups are added. From this viewpoint as well, it is anticipated that a high oxidative consumption resistance can be obtained.
If explaining the problem of flooding more specifically, at the cathode side catalyst layer, not only water molecules moving from the anode side accompanied with the protons, but also water molecules produced by the cathode side reduction reaction become added as water vapor, so the saturated steam pressure is exceeded and these condense to water. Further, the water produced by this reduction reaction becomes greater in amount at the time of large current discharge and pools in the pores at the catalyst layer serving as both the path for gas diffusion and drainage of the produced water and thereby causes clogging. There is therefore the problem that the supply and diffusion of gas to the inside of the catalyst layer become insufficient and the fuel cell falls in voltage. Further, this problem of flooding becomes a major issue at the time of commercialization in applications such as fuel cell vehicles where increase of the output voltage at the time of large current discharge for raising the maximum output is sought.
In this regard, to improve the power generation performance at the time of large current discharge (large current characteristic), it is possible to increase the speed of diffusion of the reaction gas comprised of the oxygen gas and to quickly discharge the water generated by the reduction reaction at the cathode side from the catalyst layer to prevent flooding.
Further, the former approach of increasing the gas diffusion speed more specifically comprises decreasing as much as possible the restriction on speed in the diffusion step until the reaction gas introduced into the catalyst layer reaches the catalyst metal particles. Specifically, the pores in the catalyst layer derived from the tree-branch shaped structures and other forms of the supporting carbon material and the pore structures inside the supporting carbon material in which the catalyst metal particles are present are made larger to an extent no longer restricting the speed of diffusion.
If it were possible to improve the diffusion ability of reaction gas in the catalyst layer and around the catalyst particles in this way, it would become possible to maintain sufficient activity even in a larger current region consuming a large amount of reaction gas and as a result it would become possible to form a fuel cell excellent in large current characteristic.
Further, for the latter approach of quickly discharging the produced water and the former approach of reducing the restriction on speed of diffusion of gas, it is necessary to make the supporting carbon material support the catalyst metal particles in a “highly dispersed state”. Here, the “highly dispersed state” is the state where catalyst metal particles are dispersed on the supporting carbon material by a certain distance and so as not to be separated more than necessary so as to enable diffusion of oxygen gas and drainage of water. If spatial distribution of catalyst metal particles in the catalyst layer becomes sparse, the amount of production of water per unit volume in the catalyst layer can be decreased and flooding can be suppressed and the speed of consumption of oxygen per unit volume in the catalyst layer becomes smaller and the regulation of the diffusion of gas becomes difficult.
Furthermore, the ease of flooding differs depending on the pore structure of the supporting carbon material. That is, the more the insides of the pores repel water and the smaller the pore size, the easier the flooding.
Therefore, to improve the gas diffusion ability, it is considered necessary to obtain a tree-branch shaped network and a structure having spaces formed among a large number of branches. Up to here as well, the following specific techniques for improvement and supporting a carbon material have been proposed.
PLT 2 discloses, as a supporting carbon material with a large specific surface area of pores of a nanometer size, that is, mesopores, a supporting carbon material obtained by the method of production shown below. That is, it blows acetylene gas into a solution containing a metal or metal salt to form a metal acetylide and heats the obtained metal acetylide in a vacuum to prepare a metal particle-containing intermediate in which metal particles are contained. Furthermore, it discloses heating this metal particle-containing intermediate in vacuum to make the metal particle-containing intermediate eject metal particles, washing the obtained carbon material intermediate, then heating the washed carbon material intermediate in a vacuum or in an inert gas atmosphere to produce a tree-branch shaped carbon nanostructure comprised of rod-shaped members or ring shaped members containing carbon which are branched outward (supporting carbon material).
PLT 3 proposes a carbon fiber catalyst layer containing, as means enabling improvement of the gas diffusion ability and suppression of flooding, a catalyst comprised of a carbon support on which platinum is supported, carbon fiber with an average fiber diameter of 5 to 20 μm, and a fluorine-containing ion exchange resin and and having a ratio of carbon fiber in the total of the carbon fiber and carbon support (100 mass %) of 60 to 85 mass %.
PLT 4 proposes forming a tree-branch shaped network and spaces between the branches and improving the gas diffusion ability by carbon black having a DBP oil absorption of 170 to 300 cm3/100 g, a specific surface area by the BET method of 250 to 400 m2/g, a primary particle size of 10 to 17 nm, and a total volume of pores opening at the surface with a radius of 10 to 30 nm of 0.40 to 2.0 cm3/g.